expand-region – Incrementally increase a region by semantic units. Very
useful when coding, as you can quickly select an entire block.

use-package – Although it doesn't add any new modes, it makes maintaining
your Emacs initialization files much more pleasant as all the package init
and config functionality can be kept in the same place.

As a bonus feature it can also speed up the Emacs launch time considerably,
as it loads packages on demand.

For Writers

flyspell – Spell checking that can be used inside other modes, such as
org-mode or markdown-mode. It can even be configured to check the spelling of
comments and strings within source code. The default keyboard shortcuts are a
little uncomfortable (ALT + SHIFT + $ to suggest corrections), but
otherwise it does the job well.

markdown-mode – Edit Markdown files from within Emacs. I prefer org-mode
as it has some nice keyboard shortcuts for reorganising files, but Markdown
is more widely used for README files etc.

darkroom-mode – An Emacs version of the popular "Darkroom" app. Allows you
specify a custom set of dimensions for fullscreen text editing. Great for
removing distractions.

mediawiki-mode – Takes a little bit of effort to get it all working, but
makes editing online wikis much easier.

For Bloggers

org2blog – Lets you write your blog posts using org-mode formatting, and
then publish them using xml-rpc. Works great with WordPress, but supports
other popular blogging platforms as well.

For Developers

General Tools

autocomplete-mode – Add auto-complete functionality to any mode. It's not
as powerful as Visual Studio or similar IDE's, but it gets the job done.

ecb – The Emacs Code Browser. Makes navigating software projects much
easier. Adds a sidebar with a directory browser, file list as constructs
within the current buffer.

YASnippet – Snippets and templates that can be quickly inserted into the
current file. Very helpful for speeding up common tasks, such as building
classes or creating tests for Behaviour Driven Development (using
feature-mode).

web-mode – Excellent mode for editing just about any kind of HTML. Works
great with PHP and Rails, as well as other template engines (Twig, Liquid,
Django). Also supports nifty features like collapsing elements and renaming
opening/closing tags at the same time.